Designers of lighting for recessed indoor applications have limited design choices. This is particularly the case when the designer desires a smaller lighting fixture package because the designated recessed lighting environment has a high grid ceiling or a shallow plenum depth between ceiling and adjacent floor joists. Traditionally, fluorescent lighting fixtures have been used in such recessed applications because they generate much less heat than other types of lighting units (such as high intensity discharge luminaires), making them ideal for placement in a high ceiling having a shallow plenum depth. Fluorescent fixtures also typically have horizontally oriented lamps which lie in the plane of the ceiling, usually because any other orientation would lead to visual discomfort and glare due to direct high angle radiation from the lamp, and may require louvers. However, fluorescent fixtures also provide relatively less illumination than other types of lighting fixtures, thereby requiring a greater number of units to achieve the desired lighting effect.
Another type of lighting unit is the high intensity discharge (HID) luminaire, which is often used in outdoor applications. HID units have generally proven unsuitable for indoor applications as they tend to generate a relatively large and undesirable amount of heat, especially in the context of enclosed spaces, such as a grid ceiling, and particularly in those ceilings having a relatively shallow plenum depth. Such fixtures also produce unacceptable glare.
Accordingly, when used indoors, such luminaires have generally used a white coated diffuse reflector and a flat prismatic lens (which lies in the ceiling plane) to help reduce the apparent brightness of the fixture. Unfortunately, the diffuse nature of this type of reflector causes a large reduction in reflector efficiency. Also, the radiant energy emitted by the HID lamp goes through multiple reflection bounces before it exits the luminaire. Because each reflection leads to some degree of absorption of radiation (for example, if reflectance is 80%, then the material absorbs 20% of the energy), this leads to more energy being absorbed by the luminaire housing and hence transmitted to the ceiling structure in the form of heat. Moreover, in traditional HID fixtures, in order to change the lighting distribution it is necessary to change the lamp center and/or the optical unit itself, which can be a time consuming and costly task.
Consequently, an improved luminaire assembly for indoor applications is desired. The improved luminaire assembly should be adapted for use in a high grid ceiling or a ceiling having a shallow plenum depth. The luminaire assembly should have acceptable levels of generated and absorbed heat, brightness and glare control for their designated application. The luminaire should also have relatively high efficiency and reflectance.